The switched reluctance machine is a simple electric machine with no electrical conductors or permanent magnets on the rotating part and only simple, switched coils, often carrying only unidirectional currents, on the stator. This attractive combination of a simple machine, coupled with the rapidly evolving capabilities and falling costs of power-electronic switches and control electronics has led to the continued development of switched reluctance drives.
FIG. 1 shows the principal components of a switched reluctance drive system 10. The input power supply 12 can be either a battery or rectified and filtered mains. The DC voltage provided by power supply 12 is switched across the phase windings of the machine 14 by the power converter 16 under the control of the electronic control unit 18. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive 10. As such, a simple rotor position encoder 20 is typically mounted on the machine shaft 22 to supply position signals to the controller 18 for determining the absolute angular position of the rotor. The encoder 20 can also be used to generate a speed feedback signal in the controller 18. FIG. 1 shows this being utilized to give closed-loop speed control.
FIG. 2 shows the elements of a typical four-phase switched reluctance machine 14. The machine 14 has eight salient poles 26a-h on the stator 28 and six poles 30a-f on the rotor 32. Each stator pole 26a-h carries a simple exciting coil 34a-h Opposite coils 34a and 34e, 34b and 34f, 34c and 34g, and 34d and 34h are connected to form the north/south pole pairs for the four "phases." Only one phase circuit 36 is shown for the opposite coils 34a and 34e. The opposite coils 34a and 34e are excited from a dc supply 38 through two switches or transistors (S1 and S2), and two diodes (D1 and D2) allow energy to return to the supply 38. Other switching circuits are well known in the art.
If it is desired to operate the machine as a motor, torque is developed in the machine 14 by the tendency for the magnetic circuit to adopt a configuration of minimum reluctance, i.e., for an opposing pair of rotor poles 30a and 30d, 30b and 30e, and 30c and 30f to be pulled into alignment with an excited pair of stator poles 26a-h, maximizing the inductance of the exciting coils 34a-h. By switching the phases in the appropriate sequence, the rotor 32 will continuously rotate in either direction so that torque is developed continuously in the appropriate direction. Moreover, the larger the current supplied to the coils 34a-h, the greater the torque. Conversely, if it is desired to operate the machine as a generator, the coils are excited as the motor poles move away from the stator poles. Power is then transferred from the shaft of the machine to the electrical supply.
FIG. 3 shows a rotor pole 41 approaching a stator pole 39 according to arrow 35 for the switched reluctance machine 14 of FIG. 2. FIG. 4 shows the phase circuit 47 for opposite coils 40 as partially depicted in FIG. 3. As the rotor pole 41 approaches the stator pole 39, an energization cycle commences for the phase associated with stator pole 39. When leading edge of the rotor pole 41 reaches position 36, represented by absolute rotor angle .THETA.1 ("on angle"), the phase transistors 44 are turned on, and the DC supply 42 is applied to the opposite coils 40, causing flux to build up. Consequently, the stator pole 39 attracts the rotor pole 41, thereby producing torque. When the rotor pole 41 reaches an absolute rotor position 37, represented by absolute rotor angle .THETA.2 ("freewheel angle"), only one transistor 44 is turned off. This causes the current to "freewheel" around the other transistor 44 and leads to an approximately constant-flux condition. When the rotor pole 41 reaches absolute rotor position 38, represented by absolute rotor angle .THETA.3 ("off angle"), both phase transistors 44 are held in the off state, and diodes 46 conduct, placing a voltage of reverse polarity across the winding and causing flux to decay to zero.
The on, freewheel, and off positions discussed above represent typical commutation points in the energization cycle for each phase of a switched reluctance machine. These three angles are controlled by the controller 18 to control the torque. The relationship between the angles and the torque is a nonlinear function of torque and speed. The controller 18 may interpolate from a look-up table of measured settings to obtain and update the proper commutation points for each phase of the switched reluctance machine.
Previous switched reluctance systems used a simple angular position sensor to control the energization of the motor phases. Typically, the sensor gives one pulse per phase for each energization cycle of the motor. The timing markers derived from the sensor are interpolated electronically to obtain adequate resolution. The advantage of this system is the low sensor cost. However, a substantial disadvantage exists with respect to the lack of information provided when running at slow speeds or high acceleration rates.
Absolute position encoders (e.g., resolvers, optical encoders) have been widely used in high performance drive systems for many years, particularly for position control systems. Absolute position encoders give angular resolution sufficient to remove the need for interpolation of angles, but, especially for higher resolution encoders, the absolute position encoders require more hardware, more software, or both to make adequate comparisons between the absolute rotor position and the required commutation points.
For the switched reluctance drive of FIG. 2, each phase undergoes six energization cycles per rotor revolution, one for each rotor pole. Since each energization cycle has three commutation positions (on, off, and freewheel), each phase requires eighteen comparisons between these required positions and the actual rotor position. For four-phase machine, this requires a total of seventy-two such comparisons. Such an arrangement would require seventy-two sets of, e.g., the circuitry of FIG. 5 which uses a twelve-bit resolver word. FIG. 5 includes a twelve-bit latch 49 storing a predetermined absolute rotor position, and a twelve-bit comparator 50 for comparing the predetermined absolute rotor position with the high resolution, absolute rotor position from the position encoder or resolver 48. These techniques are standard and are well known by those skilled in the art. Other implementations would be possible.